Environmental Engineering Reference
In-Depth Information
Fig. 4.10 Flow
figure of the
electrodialysis process (PCA
—
Polymerchemie Altmeier
GmbH
2014
)
the in
g-
uration called an electrodialysis cell. The cell consists of a feed (dilute) compart-
ment and a concentrate (brine) compartment formed by an anion exchange
membrane and a cation exchange membrane placed between two electrodes.
Electrodialysis processes are different compared to distillation techniques and other
membrane-based processes (such as RO) in that dissolved species are moved away
from the feed stream rather than the reverse (Fig.
4.10
).
uence of an applied electric potential difference. This is done in a con
4.3.4 Ion Exchange
The fourth method of desalination is called ion exchange. In this process unde-
sirable ions in the raw water are exchanged for desirable ions as the water passes
through granular chemicals called ion exchange resins. For example, cation
exchange resins are typically used in homes and municipal water treatment plants to
remove calcium and magnesium ions in
water, and by industries in the
production of ultra-pure water. The higher the concentration of dissolved solids in
the raw water, the more often the resins will need to be replaced or regenerated.
With rising costs for resins and for disposing of regeneration solutions, ion
exchange is now competitive with RO and electrodialysis only in treating relatively
dilute solutions containing a few hundred parts per million of dissolved solids.
“
hard
”
4.3.5 Freeze Desalination
The
fifth method of desalination is called freeze desalination. Freeze desalination is
a
five-stage process represented in Fig.
4.11
. In stage one, raw water is precooked to
remove harmful bacteria. In stage two, the water is frozen into slush, where ice
crystals are allowed to form. The ice is then removed in stage three and washed in
stage four. Once clean, the ice is melted to form fresh drinkable water. While the
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